The present invention relates to a frame structure of a motorcycle, and more particularly, to a frame structure in which a head pipe disposed in the front side of a vehicle frame, and left and right main pipes, and a down tube are connected to one another by a welding.
In general, a motorcycle includes a vehicle frame for configuring a frame thereof. A head pipe is provided in the front side of the vehicle frame so as to axially support a steering shaft and a front fork at the upper and lower ends thereof, respectively. Then, the rear portion of the head pipe is connected by a welding to a pair of left and right main pipes and a down tube formed as a single body to thereby configure a frame structure of the motorcycle.
As a related-art frame structure, for instance, as shown in FIG. 8, a main pipe 6 is welded to a cylindrical portion 2 of a head pipe 1 which is forged by separate left and right molds, upper and lower flange portions 3 and 4 which extend from the cylindrical portion 2 to the rear side, a longitudinal flange portion 5 which is connected to the rear portions of the upper and lower flange portions 3 and 4, and a down tube 9 is welded to the lower surface of the lower flange portion 4, tongue pieces 7 and 8 which are disposed below the lower flange portion 4 and which extend from the cylindrical portion 2 to the rear side in a downward inclined direction (for instance, see Patent Document 1).
Patent Document 1: Japanese Patent Publication No. 2001-10578A
However, since the related-art frame structure has a configuration in which a front wheel is axially supported by the lower end of the front fork, a pushing-up load applied from a road surface is transmitted to the head pipe through the front fork. In particular, stress is concentrated on a portion in which the lower portion of the head pipe 1 is connected to the down tube 9. Additionally, since the down tube 9 is configured by a pipe, it is not possible to partly improve strength. As a result, since it is necessary to increase a thickness of a plate or a thickness of the down tube 9 as a whole in order to configure a frame structure which endures the pushing-up load applied from the road surface, a problem arises in that it is difficult to reduce a weight of the frame structure.